The present invention relates to ink drop charging devices. More particularly, the invention relates to an ink drop charging device adapted for use with a charging binary type multi-nozzle ink jet printer.
In an ink jet printer of the general type to which the invention pertains, ink particles not used for printing are charged and the ink particles thus charged deflected in a direction perpendicular to the direction of the ink jet while passing through a deflecting field which is created by deflecting electrodes as a result of which the charged ink particles or droplets are removed from the primary or printing stream and collected in a gutter. Ink particles used for printing are not charged. The uncharged ink particles move straightly through the deflecting field to the recording medium and are used to form recorded images or characters in the form of dots.
The charging electrode array is made up of a plurality of electrodes which are electrically insulated from one another. Each electrode is in the form of a tunnel so as to charge the corresponding ink jet and to prevent the occurrence of interference with adjacent electrodes. That is, the ink jet flows along the central axis of the tunnel-shaped electrode.
A charging signal applied to the charging electrode has the same potential as that of the ink jet, usually, the ground potential, for the ink droplets used for printing. On the other hand, a positive or negative potential is applied to ink droplets which are not used for printing and are recovered by the gutter. Application of these potentials is effected when an ink droplet is about to leave the ink jet.
FIG. 1 illustrates the relationship between the charging signals and the ink drops. An ink stream 2 jetted by a nozzle 1 is broken into ink droplets 3, 4, 5 and 6 successively by action of a piezoelectric element (not shown) which oscillates in synchronization with the charging signals. The velocity of the ink droplets is sufficiently high that no special synchronization techniques are required and the piezoelectric element can be driven with the charging signal. The ink droplets 3 through 6 are charged or not charged depending upon the state of the charging signal during pulse periods 3A through 6A. For instance, during pulse period 5A in which the signal is at ground potential, the ink droplet 5 is not charged while during the pulse period 6A in which the signal is at a negative potential such as 100 V, the ink droplet 6 is charged positive. Thus, the ink jet and the jetted ink drops are charged positive or not charged and the sum of the charges on the ink droplets is positive.
The structure of the charging electrode 10 described above is shown in FIG. 2. An electrode portion 12 is formed on an insulating substrate 11. In order to prevent the occurrence of a short-circuit should the electrode portions 12 be wet, the electrode portion is completely covered with an insulating film 13. In FIG. 2, the charging electrode 10 is at a negative potential and therefore positive charges are accumulated on the surface of the ink 2.
If, under this condition, a foreign particle 151 such as a dust particle floating in the air is brought into contact with the ink, the particle will be charged by the ink 2. That is, the particle is charged positive. The particle 152 thus charged positive is attracted towards the charging electrode 10 by the electrostatic field created between the ink and the charging electrode and is held by electrostatic force upon the surface of the insulating film 13.
at this process continues, the charging electrode will become completely charged up. As a result, the ink droplets cannot be correctly charged because of the charges which are created on the charging electrode by the accumulated particles. This phenomenon becomes more significant in the case where the inside of the charging tunnel in the charging electrode is dirty because in this case the charged floating particles can be readily caught by the dirty charging tunnel.
Accordingly, an object of the present invention is to provide an ink droplet charging device in which charging up of the charging electrode is prevented whereby ink droplets are correctly charged at all times.